[Elecraft] [K3-KPA500] SWR jumps up with 5 second key down

[Ron D'Eau Claire]

Mark has an excellent point, especially if you have a ferrite core
transformer (balun, etc.) somewhere in the antenna system. One thing that
can cause that behavior is heating of the toroid core. At a certain
temperature, the core's magnetic properties change dramatically, changing
the inductance and so causing the SWR to change. 

Ron AC7AC

-----Original Message-----

What happens when you run the KPA-500 into a dummy load? If all is well, I
would guess that something is changing under high power and affecting the
load.

-----Original Message-----
 From: ai6ii <ai6ii at att.net>
 To: elecraft at mailman.qth.net
 Date: Mon, 4 Jun 2012 10:00:54 -0700 (PDT)
 Subject: [Elecraft] [K3-KPA500] SWR jumps up with 5 second key down


I have installed a new antenna and have strange changing swr. When testing
with RigExperts meter I find good resonance at say 15m (21.200). With the
K3
 at 100 watts reading is 1 bar on K3 meter and 1.0 on KPA500 (STBY). When I
bring the amp online and drive it at 200 watts, all is still good even with
a 15 second key down period. Driving the KPA500 to 275 watts, still okay
with key down, although the KPA500 meter now shows 1.1. BUT when I increase
the drive a little more (300 watts) the swr jumps up to 2.5+ and the KPA500
shows REFL HI and drops into STBY.  

 And here is the kicker...NOT ALL THE TIME!

 So I think it has to be in the coax. But the 75feet of RG213 is brand new
premade from Cable Xperts with the connectors.

 I will try a different coax. Any other suggestions?

 ..mike